Under the Moore Law, the integration density of conventional Si-based MOS device is continually raised, while it is also faced various challenge and limitation, such as, mobility degradation, carrier velocity saturation and DIBL effect and so on, in which the mobility degradation becomes one of the critical factors affecting the of further improvement of device performance. In order to solve these problems resulting from the scaling down of the device, it is necessary to use a channel material with a high mobility. At present, Ge-based Schottky MOS transistor becomes one of the hot research focuses. Firstly, the electron and hole mobility of the Ge material are higher than that of silicon material, and the fabrication process of the Ge channel device is compatible with the conventional CMOS process. Secondly, the substitution of the conventional highly doped source/drain with a Schottky source/drain structure not only avoids the problems of low solid solubility and rapid diffusion of impurities in the Ge material, but also decreases resistances of the source/drain. Therefore, the Ge-based Schottky MOS transistor is promising to break the limitation of the conventional Si-based device, and thus an excellent device performance can be obtained.
However, the Ge-based Schottky MOS transistor also has some problems to be solved. There are a large number of dangling bonds at the interface between the source/drain and a substrate of the Ge-based schottky MOS transistor, and the Fermi energy level is pinned at the vicinity of the valence band due to a Metal-Induced Gap State (MIGS) generated by metal (or metal germanide) in the Ge forbidden band, leading to a high electron barrier. The high electron barrier severely limits the improvement of the performance of the Ge-based Schottky MOS transistor. A higher barrier of the source/channel in on-state limits a current drive ability of the device, and a lower barrier of the source/channel in off-state leads to increasing of a leakage current of the device. Meanwhile, the high electron barrier causes the electrons enter into the channel mainly by means of tunneling, so that a subthreshold slope of the device is increased. Therefore, the electron barrier height becomes one of the critical factors affecting the performance of the Ge-based Schottky NMOS transistor.